Earth clocks up its shortest day . . . out of sync with heavens above
If it feels like there is never enough time in the day, there may be a reason.
Last month Earth had its shortest day since records began, with 1.59 milliseconds shaved off the usual 24 hour spin on June 29 — raising the prospect that a negative leap second may soon be needed to keep clocks matched up with the heavens.
Usually, Earth’s average rotational speed decreases slightly over time and timekeepers have been forced to add 27 leap seconds to atomic time since the 1970s as the planet slows.
But since 2020, the phenomenon has reversed with records being frequently broken over the last two years. The previous fastest day was -1.47 milliseconds under 24 hours on July 19 2020 and it was almost broken again on July 26, when the day was -1.50 milliseconds shorter. While the effect is too small to be noticeable by humans, it can accumulate over time, potentially impacting modern satellite communication and navigation systems which rely on time being consistent with the conventional positions of the Sun, Moon and stars.
It may soon be necessary to remove time, adding a negative leap second, and speeding up global clocks for the first time ever.
Experts have suggested that a phenomenon known as the “Chandler Wobble” may be having an impact. The speed of the Earth’s rotation varies constantly because of the complex motion of its molten core, oceans and atmosphere, as well as the effect of celestial bodies such as the Moon.
The friction of the tides and the change in distance between the Earth and the Moon all make for daily variations in the speed the planet rotates on its axis. The Chandler Wobble is the change in the spin of the Earth on its axis and it normally causes Earth’s rotation to increase, meaning it takes longer to complete a turn. But in recent years the spin has become less wobbly.
Dr Leonid Zotov, of the Sternberg Astronomical Institute, believes this lack of wobble may be behind the speedier days. He will present the theory next week at the Asia Oceania Geosciences Society annual meeting.
“The normal amplitude of the Chandler Wobble is about 3m to 4m at Earth’s surface, but from 2017 to 2020 it disappeared,” Zotov told the website Timeanddate.
In the early 2000s, the amplitude of the Chandler wobble started to decrease and in 2017-2020 reached a historic minimum just as the length of day began to shorten. Other factors that can have an impact include snow building on Northern Hemisphere mountains and then melting.
Global warming is also expected to have an effect by melting ice and snow at higher elevations, causing the Earth to spin faster, but it’s considered to be a relatively small contribution.
Changes to the length of a standard day were only discovered after highly accurate atomic clocks were developed in the 1960s and compared to fixed stars in the sky.
The last leap second was added on New Year’s Eve 2016, when clocks around the world paused for a second to allow the Earth’s rotation to catch up.
The International Earth Rotation Service, based in Paris, monitors the planet’s rotation and informs countries when leap seconds must be added or taken away six months in advance.
However the leap second could be abolished entirely next year, when the World Radiocommunication Conference will decide whether to rely completely on atomic time. Britain is opposed to the move because it would sever the link with solar time forever.